Basic Error Handling

When something goes wrong while a program runs — dividing by zero, converting "abc" to a number — Ruby raises an exception. If nothing handles it, the program stops and prints the error.

Real-world analogy: an exception is a fire alarm. rescue is the plan for what to do when it goes off, instead of everyone panicking.

Syntax

begin
  # code that might fail
rescue SomeError => e
  # what to do if it does
end

Example

def safe_divide(a, b)
  a / b
rescue ZeroDivisionError
  puts "You can't divide by zero."
  nil
end

p safe_divide(10, 2)
p safe_divide(10, 0)

Expected output

5
You can't divide by zero.
nil

Inside a method, you can put rescue directly before the method's end — no begin needed.

Using the error

=> e stores the exception in a variable, so you can read its message:

input = "abc"

begin
  number = Integer(input)
  puts "You entered #{number}"
rescue ArgumentError => e
  puts "Invalid number: #{e.message}"
end

Invalid number: invalid value for Integer(): "abc"

Common built-in errors you'll meet: ArgumentError (bad argument), TypeError (wrong type), NoMethodError (method doesn't exist — often called on nil), ZeroDivisionError, and KeyError (from fetch).

Level 2 covers exceptions in depth: ensure, retry, raising your own errors, and custom error classes.

Common mistakes

• Rescuing everything with a bare rescue and hiding real bugs. Always name the specific error you expect.
• Using exceptions for normal decisions. If you can check first (b.zero?), that's usually clearer.

Practice

Write a method parse_age(text) that returns the age as an integer, or prints Please enter a whole number and returns nil if the text isn't a valid integer.

def parse_age(text)
  Integer(text)
rescue ArgumentError
  puts "Please enter a whole number"
  nil
end

p parse_age("42")
p parse_age("forty")

42
Please enter a whole number
nil